With reference to FIGS. 1 and 2, a conventional shock absorber is shown to include a base unit 100, a primary cylinder 2, a hollow shock rod 22, a main piston 221, a biasing member 23, a hydraulic adjusting unit 24, a secondary cylinder 3, and a floating piston 32. The main piston 221 is connected to the hollow shock rod 22, and is slidably disposed in a primary chamber 21 of the primary cylinder 2 to divide the primary chamber 21 into two sub-chambers 201, 202. The main piston 221 has two first passageways 222 and a second passageway 223, each interconnecting the two sub-chambers 201, 202. The biasing member 23 is a biasing spring and is disposed to bias an impact side end 220 of the hollow shock rod 22 away from the primary cylinder 2. The hydraulic adjusting unit 24 has an inner rod 241 which is movably and rotatably engaged inside the hollow shock rod 22, and which is configured to be moved relative to the hollow shock rod 22, thereby adjusting flowing of a working liquid passing through the secondary passageway 223. The floating piston 32 is disposed slidably in a secondary chamber 31 of the secondary cylinder 3 to divide the secondary chamber 31 into a liquid sub-chamber 311 and a gas sub-chamber 312. When an impact force is exerted on the hollow shock rod 22 against a biasing force of the biasing member 23 to force the working liquid in the primary chamber 21 to flow into the liquid sub-chamber 311, a working gas in the gas sub-chamber 312 is compressed to generate a damping force to act on the working liquid so as to permit the working liquid in the liquid sub-chamber 311 to flow back to the primary chamber 21. The damping characteristic inside the primary cylinder 2 can be adjusted by the hydraulic adjusting unit 24. However, the damping characteristic inside the secondary cylinder 3 is fixed and cannot be adjusted. Therefore, it is difficult for a car driver to adjust the damping force of the conventional shock absorber based on different road surface conditions.